Combined use of ionic liquid and sulfobutylether‐β‐cyclodextrin as electrolyte additives for separation and determination of camptothecin alkaloids by CZE

This work reported that ionic liquid (IL) ([Bmim] [PF₆]) and sulfobutylether‐β‐CD (SBE‐β‐CD) were used as electrolyte additives for the separation and determination of camptothecin (CPT) alkaloids by CZE. Separation parameters such as the buffer type, pH, and concentration of the running buffer, the concentration of SBE‐β‐CD and IL, temperature, and separation voltage were all investigated in order to achieve the maximum possible resolution. The four analytes were baseline separated within 10 min in capillary at the separation voltage of 15 kV with a running buffer consisting of 20 mM borate buffer, 20 mM IL, and 100 mM SBE‐β‐CD at pH 9.0. Under such conditions, good linearity about two orders of magnitudes of peak areas was achieved for the investigated CPT alkaloids with the correlation coefficients ranging from 0.9946 to 0.9985. For all analytes, detection limits (S/N = 3) and quantitation limits (S/N = 10) range from 0.05 to 0.92 μg/mL and 0.17 to 3.06 μg/mL, respectively. The proposed method has not only been successfully applied to the separation and determination of CPT alkaloids but also showed that IL seemed to be a promising additive in CZE separation.     

On-line pretreatment and determination of parabens in cosmetic products by combination of flow injection analysis, solid-phase extraction and micellar electrokinetic chromatography

A convenient and automated method for on-line pretreatment and determination of three parabens (i.e. methyl, ethyl and propyl p-hydroxybenzoate) in cosmetic products is proposed by using flow injection analysis (FIA), solid-phase extraction (SPE) and micellar electrokinetic chromatography (MEKC). An improved split–flow interface is used to couple SPE on C₈-bonded silica with MEKC separation, which can avoid running buffer contamination and reduce buffer consumption, especially, containing expensive reagents. The analytes are loaded onto a C₈ column at 0.6 mL/min for 60 s and eluted with a mixed eluent of 40% (v/v) 10 mmol/L sodium tetraborate buffer (pH 9.3) and 60% (v/v) ethanol at 0.75 mL/min. The MEKC separation is accomplished with a running buffer of 20 mmol/L sodium tetraborate (pH 9.3) containing 100 mmol/L sodium dodecyl sulfate (SDS) at 15 kV. For butyl p-hydroxybenzoate did not be detected in the cosmetic products, it was used as an internal standard (IS) added into the real samples. This FIA–SPE–MEKC method using IS allows the sample separation within 12 min and the sample throughput of five samples per hour with the relative standard deviation (R.S.D.) less than 2.3% (n = 5). The limits of detection (LOD) are in the range from 0.07 to 0.1 μg/mL (S/N = 3 and n = 11). The proposed method has been used to determine three parabens in real cosmetic products satisfactorily.     

电动流动分析-胶束电动毛细管色谱联用测定食品防腐剂

采用电动流动分析(EFA)和胶束电动毛细管色谱(MEKC)联用系统测定了食品中的防腐剂.EFA系统设备简单便携、易于实现自动化.EFA由一台自制电渗泵、5个电磁切换阀(由计算机通过自制接口卡和自编VC语言程序控制)和一个固相萃取微柱组成,采用EFA-MEKC低试剂消耗接口,可减少含贵重试剂的缓冲液消耗.MEKC缓冲溶液为100 mmol/L十二烷基硫酸钠(SDS) 20 mmol/L Na2B4O7(pH 9.3).使用对羟基苯甲酸(PBA)为内标物,方法可在15 min内分离测定6种防腐剂,峰面积相对标准偏差小于3.4%;检出限范围为0.04～0.1 mg/L;实际样品的回收率为91.4%～104%.     

Micellar electrophoresis using ionic liquids

In this study, ionic liquid based cationic surfactants were evaluated as pseudo-stationary phases in micellar electrokinetic chromatography (MEKC). The aggregation behaviour of long-chain (C(12) and C(14)) alkylimidazolium ionic liquids in water and aqueous phosphate buffer was investigated by spectrophotometry. The critical micelle concentrations of these salts were determined and compared to those of tetradecyl- and dodecyltrimethylammonium chloride, salts commonly used in capillary electrophoresis. The practical utilization of a new type of surfactant in MEKC was evaluated by introducing an ionic liquid into the running aqueous buffer to separate neutral analytes-methylresorcinol isomers and benzene derivatives.     

Capillary electrochromatography with zwitterionic stationary phase on the lysine-bonded poly(glycidyl methacrylate-co-ethylene dimethacrylate) monolithic capillary column

A polymer-based neutral monolithic capillary column was prepared by radical polymerization of glycidyl methacrylate and ethylene dimethacrylate in a 100 mum id fused-silica capillary, and the prepared monolithic column was subsequently modified based on a ring opening reaction of epoxide groups with 1 M lysine in solution (pH 8.0) at 75 degrees C for 10 h to produce a lysine chemically bonded stationary phases in capillary column. The ring opening reaction conditions were optimized so that the column could generate substantial EOF. Due to the zwitterionic functional groups of the lysine covalently bonded on the polymer monolithic rod, the prepared column can generate cathodic and anodic EOF by varying the pH values of running buffer during CEC separation. EOF reached the maximum of -2.0 x 10(-8) m2v(-1)s(-1) and 2.6 x 10(-8) m2v(-1)s(-1) with pH of the running buffer of 2.25 and 10, respectively. As a consequence, neutral compounds, ionic solutes such as phenols, aromatic acids, anilines, and basic pharmaceuticals were all successfully separated on the column by CEC. Hydrophobic interaction is responsible for separation of neutral analytes. In addition, the electrostatic and hydrophobic interaction and the electrophoretic migration play a significant role in separation of the ionic or ionizable analytes.     

Separation of caffeine and theophylline in poly(dimethylsiloxane) microchannel electrophoresis with electrochemical detection

A method for the rapid separation and sensitive determination of caffeine and theophylline was presented in poly(dimethylsiloxane) (PDMS) microchannel electrophoresis integrated with electrochemical detection. By using methanol as an additive, the peak shape and resolution were essentially improved. The analytes were well separated within only 40s in the running buffer of 5.0mM borate solution (pH 9.2) containing 10% (v/v) methanol. The linear ranges were from 6microM to 0.6mM and the detection limits were 4microM for caffeine and theophylline, respectively. The proposed method has been successfully applied to determine caffeine and theophylline in rat serum and urine.     

Comparing micellar electrokinetic chromatography and microemulsion electrokinetic chromatography for the analysis of preservatives in pharmaceutical and cosmetic products

In this study, separation and determination of nine preservatives ranging from hydrophilic to hydrophobic properties, which are commonly used as additives in various pharmaceutical and cosmetic products, by micellar electrokinetic chromatograpy (MEKC) and microemulsion electrokinetic chromatography (MEEKC) were compared. The effect of temperature, buffer pH, and concentration of surfactant on separation were examined. In MEKC, the separation resolution of preservatives improved markedly by changing the sodium dodecyl sulfate concentration. Temperature and pH of running buffers were used mainly to shorten the magnitude of separation time. However, in order to detect all preservatives in a single run in a MEEKC system, a microemulsion of higher pH was needed. The separation resolution was improved dramatically by changing temperature, and a higher concentration of SDS was necessary for maintaining a stable microemulsion solution, therefore the separation of the nine preservatives in MEEKC took longer than in MEKC. An optimum MEKC method for separation of the nine preservatives was obtained within 9.0 min with a running buffer of pH 9.0 containing 20 mM SDS at 25 degrees C. A separation with baseline resolution was also obtained within 16 min using a microemulsion of pH 9.5 which composed of SDS, 1-butanol, and octane, and a shorter capillary column at 34 degrees C. Finally, the developed MEKC and MEEKC methods determined successfully preservatives in various cosmetic and pharmaceutical products.     

Use of short chain alkyl imidazolium ionic liquids for on‐line stacking and sweeping of methotrexate,flinic acid and folic acid: Their application to biological fluids

Methotrexate (MTX) is widely used for the treatment of many types of cancer. Folinic acid (FNA) and folic acid (FA) were usually simultaneously supplemented with MTX to reduce the side effects of a folate deficiency. This study, for the first time, included on‐line sample preconcentration by stacking and sweeping techniques under reduced or enhanced electric conductivity in the sample region using short chain alkyl imidazolium ionic liquids (ILs) as micelle forming agents for analyte focusing. Both analyte focusing by micelle collapse (AFMC) and sweeping‐MEKC had been investigated for the comparison of their effectiveness to examine simultaneously MTX, FNA and FA in plasma and urine under physiological conditions. In sweeping‐MEKC, the sample solution without micelles was hydrodynamically injected as a long plug into a fused‐silica capillary pre‐filled with phosphate buffer containing 3.0 mol/L of 1‐butyl‐3‐methylimidazolium bromide (BMIMBr). Using AFMC, the analytes were prepared in BMIMBr micellar matrix and hydrodynamically injected into the phosphate buffer without IL micelles. The conductivity ratio between BGE and sample (γ, BGE/sample) was optimized to be 3.0 in sweeping‐MEKC and 0.33 in AFMC resulting the adequate separation of analytes within 4.0 min. To reduce the possibility of BMIMBr adsorption, an appropriate rinsing protocol was used. The limits of detection were calculated as 0.1 ng/mL MTX, 0.05 ng/mL FNA and 0.05 ng/mL FA by sweeping‐MEKC and 0.5 ng/mL MTX, 0.3 ng/mL FNA and 0.3 ng/mL FA by AFMC. The accuracy was tested by recovery in plasma and urine matrices giving values ranging between 90 and 110%. Both stacking and sweeping by BMIMBr could be successfully used for the rapid, selective and sensitive determination of pharmaceuticals in complex matrices due to its fascinating properties, including high conductivity, good thermal stability and ability to form different types of interactions by electrostatic, hydrophobic, hydrogen bonding and π–π interactions. In sweeping‐MEKC, the using of BMIMBr enhanced the γ factor, k retention factor and the injected amount of sample. Consequently, this technique offers particular potential for higher sensitivity by giving 22‐ and 5‐fold sensitivity enhancement factors (SEFs) of MTX compared to CZE and AFMC, respectively.     

Determination of polyphenol components in herbal medicines by micellar electrokinetic capillary chromatography with Tween 20

A simple and convenient method of micellar electrokinetic capillary chromatography (MEKC) using polyoxyethylene sorbitan monolaurate (Tween 20) to form single micelle and methanol as a buffer additive was introduced for the simultaneous determination of five polyphenols, including scopoletin, rutin, esculetin, chlorogenic acid and caffeic acid. A running buffer solution of pH 9.3, 20 mmol/L sodium tetraborate containing 64 mmol/L Tween 20 and 9% (v/v) methanol was adopted in the separation. Because rutin and esculetin were difficult to be separated by capillary zone electrophoresis (CZE) and SDS-based MEKC, Tween 20-based MEKC was adopted and the polyphenols were separated satisfactorily. The proposed method was used to determine the polyphenol components in the herbal medicine of Cortex fraxini. The separation mechanism of Tween 20-based MEKC for the polyphenols was discussed preliminarily.     

Micellar electrokinetic capillary chromatography determination of zinc bacitracin and nystatin in animal feed

An MEKC procedure was developed for the separation of zinc bacitracin (Zn-BC) and nystatin (NYS) in mixtures and in animal feedstuff. The running buffer was 15 mM borate/19 mM phosphate, pH 8.2, containing 20 mM SDS and 10% v/v methanol. Samples were run at 25 degrees C, the applied voltage was 25 kV, and an additional pressure of 5 mbar was applied. Both analytes were detected by UV simultaneously at 215 nm, Zn-BC alone at 192 and 254 nm, and NYS alone at 305 nm. The method was shown to be specific, accurate (recoveries were 100.0 +/- 0.6% and 100.1 +/- 0.6% for Zn-BC and NYS, respectively), linear over the tested range (correlation coefficients 0.9991 and 0.9994), and precise (RSD below 1.3% for both analytes). The method was applied to determine Zn-BC and NYS as additives in animal feed.     

Separation and determination of haloperidol, parabens and some of their degradation products by micellar electrokinetic chromatography

A micellar solution containing phosphate buffer, anionic surfactant, and water-miscible organic solvent was employed as a migration solution for the separation and the quantification of eleven analytes by micellar electrokinetic chromatography (MEKC): the analytes examined were haloperidol, methylparaben, ethylparaben, n-propylparaben, iso-propylparaben, n-butylparaben, iso-butylparaben, sec-butylparaben, 4-(4-chlorophenyl)-4-hydroxypiperidine, 4-fluorobenzoic acid and 4-hydroxybenzoic acid. In order to provide good separation between micelle and haloperidol, which showed strongest interaction with the micelle among the analytes, surfactant concentrations and organic modifier percentages were studied with phosphate buffer at pH 7.0. All the analytes were successfully resolved when 10 mM sodium dodecylsulfate and 15% ethanol were contained in the migration solution; the time window was very wide in the range from 14.8 to 65.5 min. Optimized applied voltage at 30 kV and capillary temperature at 45 degrees C enable analyze all compounds in less than 17 min with the best resolution, the shorter migration time window, the highest precision and lowest detection limit.     

A comparison of ionic liquids to molecular organic solvents as additives for chiral separations in micellar electrokinetic chromatography

In this study, we report the effects of adding ionic liquids (ILs), as compared to adding conventional molecular organic solvents (MOSs), to aqueous buffer solutions containing molecular micelles in the separation of chiral analyte mixtures in micellar EKC (MEKC). The molecular micelle used in this study was polysodium oleyl-L-leucylvalinate (poly-L-SOLV). The ILs were 1-alkyl-3-methylimidazolium tetrafluoroborate, where the alkyl group was ethyl, butyl, hexyl, or octyl. These ILs were chosen due to their hydrophobicity, good solvating, and electrolyte properties. Thus, it was expected that these ILs would have favorable interactions with chiral analytes and not adversely affect the background current. Common CE buffers, mixed with a molecular micelle, and an IL or a MOS, were used for these chiral separations. The buffers containing an IL in the concentration range of 0.02-0.1 v/v were found to support a reasonable current when an electric field strength of 500 V/cm was applied across the capillary. However, a current break down was observed for the buffers containing more than 60% v/v MOS on application of the above-mentioned electric field. The chiral resolution and selectivity of the analytes were dependent on the concentration and type of IL or MOS used.     

Electrochemical pH oscillations of ethyl viologen/ionic liquid

A reversible and robust electrochemical pH oscillator was achieved using an ethyl viologen/ionic liquid (IL) aqueous solution under an applied redox potential in a batch reactor, where the IL incorporated into the pH oscillator increased the stability of the pH oscillation by acting as an electron buffer solution.     

Separation and selectivity in micellar electrokinetic chromatography using sodium dodecyl sulfate micelles or Tween 20-modified mixed micelles

The separation and selectivity of eight aromatic compounds ranging from hydrophilic to hydrophobic properties in micellar electrokinetic chromatography (MEKC) using sodium dodecyl sulfate (SDS) micelles or Tween 20-modified mixed micelles were investigated. The effect of different operation conditions such as SDS and Tween 20 modifier surfactant concentration, buffer pH, and applied voltage was studied. The resolution and selectivity of analytes could be markedly affected by changing the SDS micelle concentration or Tween 20 content in the mixed micelles. Applied voltage and pH of running buffers were used mainly to shorten the separation time. Complete separation of eight analytes could be achieved with an appropriate choice of the concentration of SDS micelles or Tween 20-modified mixed micelles. Quicker elution and better precision could be obtained with SDS-Tween 20 mixed micelles than with SDS micelles. The mechanisms that migration order of those analytes was mainly based on their structures and solute-micelle interactions, including hydrophobic, electrostatic, and hydrogen bonding interactions, were discussed.     

Modified micellar electrokinetic chromatography in the analysis of catechins and xanthines in chocolate

Modified micellar electrokinetic chromatography (MEKC) analysis of monomeric flavanols (catechin and epicatechin) and methylxanthines (caffeine and theobromine) in chocolate and cocoa was performed by using sodium dodecyl sulfate (SDS) as a principal component of the running buffer. Because of the reported poor stability of catechins in alkaline solutions, acidic conditions (pH 2.5) were chosen and consequently the electroosmotic flow (EOF) was significantly suppressed; this resulted in a fast anodic migration of the analytes partitioned into the SDS micelles. Under these conditions, variations of either pH value in acidic range or SDS concentration, showed to be not suitable to modulate the selectivity. To overcome this limit, use of additives to the SDS-based running buffer was successfully applied and three different systems were optimized for the separation of (+)-catechin, (-)-epicatechin, caffeine, and theobromine in chocolate and cocoa powder samples. In particular, two mixed micelle systems were applied; the first consisted of a mixture of SDS and 3-[(3-cholamidopropyl)dimethylammonio]-1-propansulfonate (CHAPS) with a composition of 90 mM and 10 mM, respectively; the second was SDS and taurodeoxycholic acid sodium salt (TDC) with a composition of 70 mM and 30 mM, respectively. A further MEKC approach was developed by addition of 10 mM hydroxypropyl-beta-cyclodextrin (HP-beta-CD) to the SDS solution (90 mM); it provided a useful cyclodextrin(CD)-modified MEKC. By applying the optimized conditions, different separation profiles of the flavanols and methylxanthines were obtained showing interesting potential of these combined systems; their integrated application showed to be useful for the identification of the low level of (+)-catechin in certain real samples. The CD-MEKC approach was validated and applied to the determination of catechins and methylxanthines in aqueous extracts from four different commercial chocolate types (black and milk) and two cocoa powders.     

Ionic liquid-assisted PDMS microchannel modification for efficiently resolving fluorescent dye and protein adsorption

Herein, a hybrid system consisting of ionic liquid (IL) and nonionic surfactant has been successfully developed for dynamic modification of PDMS microchips and analyte adsorption such as fluoresent dyes and proteins has been efficiently suppressed. Mutual authentication between microchip electrophoresis and confocal laser scanning microscope was carried out to characterize the multiple novel functions of the IL-containing system and a possible mechanism was raised. Soluble IL used herein not only played the role as supporting electrolyte, but also provided increased EOF in the PDMS microchannel compared with common electrolytes such as phosphate buffer. Due to the high ionic conductivity of IL, on-column field-amplified sample stacking effect was four-fold higher than that without IL. Furthermore, an excellent synergistic effect existed between IL and nonionic surfactant, which enhanced the ability of resolving analyte adsorption to PDMS surface, and was demonstrated in the sensitive and efficient determination of rhodamine B (with detection limit of 8 nM) and a well separated mixture of proteins.     

Analysis of ecdysteroids by micellar electrokinetic chromatography with on-line preconcentration

In order to enhance the UV detection sensitivity, an application study of an on-line preconcentration technique for micellar electrokinetic chromatographic (MEKC) was carried out. The simultaneous determination of four test ecdysteroids, 20-hydroxyecdysone, ajugasterone C, polypodine B and ponasterone A has been investigated by using the normal stacking mode in MEKC with UV detection. The effects of anionic surfactant composition and concentration, the applied voltage, the pH buffer, the kind and the amount of organic solvent and the injection time on the analyte resolution were evaluated. The optimised conditions for the separation involved the use of a 50 mM borate as the running buffer containing 50 mM of a mixture of sodium dodecyl sulphate (SDS) and sodium cholate (SC) in the ratio of 1:1 together with a concentration of 10% (v/v) of 2-PrOH at pH 9.0. Hydrodynamic injection of 12 s at 50 mbar and separation voltage of 20 kV at temperature of 20 degrees C were employed. These conditions allowed a repeatability separation within 21 min. Concentration detection limit for the neutral analytes studied improve about an order of magnitude. The method was also applied to the determination of ecdysteroids in a real sample.     

Poly(dimethylsiloxane)‐Supported Ionogels with a High Ionic Liquid Loading

The immiscibility of poly(dimethylsiloxane) (PDMS) and ionic liquids (ILs) was overcome to create PDMS‐supported IL gels (ionogels) with IL loadings of up to 80 % by mass through a simple sol–gel reaction at room temperature. By stirring a mixture of a functionalized PDMS oligomer, formic acid, and an IL (or lithium‐in‐IL solution), a resin was formed that could be cast to create a freestanding, flexible ionogel. PDMS‐supported ionogels exhibited favorable ionic conductivity (ca. 3 mS cm^?1) and excellent mechanical behavior (elastic modulus: ca. 60 kPa; fatigue life: >5000 cycles; mechanically stable at temperatures up to 200 °C). The activation energy of ionic conductivity was shown to be nearly identical for the ionogel and the neat IL, in contrast to ionogel systems wherein the scaffold material is miscible with the IL. This similarity indicates that IL/scaffold chemical interactions are key to the understanding of ionogel electrical performance, especially at elevated temperatures.     

Analysis of organic components of smokeless gunpowders: high-performance liquid chromatogaphy vs. micellar electrokinetic capillary chromatography

The aim of the present study was to verify the analytical performances of high-performance liquid chromatography (HPLC) and micellar electrokinetic capillary chromatography (MEKC) for the separation and qualitative determination of a selected group of organic components of smokeless gunpowders. The HPLC method was based on a gradient reversed-phase elution with a mobile phase composed of 0.17 M H(3)PO(4)/methanol; detection was performed by UV absorption at the wavelengths of 220, 254, and 270 nm. The MEKC experiments were carried out by using uncoated fused-silica capillaries (50 microm inside diameter, 50 cm effective length) and a running buffer composed of 10 mM sodium tetraborate at pH 9.24 added with 25 mM sodium dodecyl sulfate (SDS); the applied voltage was 25 kV; detection was either at a fixed wavelength UV of 214 nm or with a diode-array detector operating in the wavelength range from 190 to 350 nm. Both reversed-phase HPLC and MEKC techniques succeeded in resolving the tested standard mixtures of organic components of smokeless powders. Although the sequence of elution of the different analytes was slightly different between HPLC and MEKC, a statistical analysis based on the Spearman's rank correlation test showed that the two separation patterns were highly correlated. HPLC and MEKC were comparable in terms of elution/migration time precision, whereas MEKC showed higher reproducibility of peak areas. The interfacing of capillary electrophoresis with diode array UV detection provided distinct UV spectra of the individual analytes, thus improving, on the detection side, the analytical selectivity and identification power of capillary electrophoresis.     

Twenty-five years of micellar electrokinetic chromatography

Micellar electrokinetic chromatography (MEKC), which can separate neutral analytes as well as charged analytes by the capillary electrophoretic technique, was developed in 1982 and the first paper was published in 1984. The authors’ group concentrated their effort into the characterization of MEKC as a separation technique until early 1990s. Most issues in MEKC separations were successfully solved and wide applicability of MEKC was verified in 1990s. In particular, sweeping, an on-line sample preconcentration technique, was very successful for the concentration of neutral analyte as well as ionic ones. In this paper, our studies on MEKC will be summarized from the personal viewpoint of the author.